Interactions between schistosomes and their snail hosts play an important role in the development of the host and parasite genetic systems. The extent of genetic heterogeneity in local parasite populations has implications on the epidemiology of host disease. Schistosoma mansoni exhibits significant phenotypic differences within and among geographic populations, yet the extent and nature of genetic variability residing in this parasite and its host snails remains largely unknown. A combination of existing multi-locus DNA markers and forthcoming single locus molecular probes (microsatellites) will be used to quantify the degree of host and parasite genetic heterogeneity in natural populations; further, we will determine how that variation changes on a spatial and temporal scale. Specific aims include: 1) determination of how genetic variation is partitioned among the various parasite and snail host subpopulations at each transmission site; and 2) assessment of relative genetic structure within and between populations of both snail host and schistosomes, and quantitative comparison of temporal genetic changes in schistosomes following chemotherapy. Results will yield evolutionary insights into the epidemiological process, help identify genetic consequences of control strategies, and complement concurrent immuno-epidemiology studies of humans in these Brazilian communities.